Chenged LED_PIN for Melzi boards, added comment about broken versions of...

... the Sanguino libraries
I know it's a trivial change since the led currently isn't used in the firmware, but it might save some troubleshooting when someone does implement a feature using it.
The pin was verified with the sample code from http://reprap.org/wiki/RepRapPro_Mendel_maintenance#Simple_Melzi_Test
using Arduino 1.0.3 and the Sanguino libraries provided with Marlin. See #368 for some info on the issue with broken libs